The Saudi Aramco Shell Refinery Company (SASREF) in Jubail shut its hydrotreater unit about two or three days ago due to a "glitch" two sources familiar with the matter said. But details of the hydrotreater's capacity or the reason for the shutdown were not clear.

Hydrotreaters are used to remove sulphur from high sulphur gasoil to make it a more environmentally friendly fuel.

Saudi Aramco offered about 60,000 tonnes of 0.5 percent sulphur gasoil from the Jubail refinery for loading over August 18-20 through private negotiations, traders said.

Bids for the cargo have to be submitted by Wednesday, they added.

Saudi Arabia is a net importer of lower sulphur diesel during summer when the fuel is used for power generation. The country has bought at least 750,000 tonnes for the whole of June and July this year.

SASREF, which is a joint venture between Saudi Aramco and Shell Saudi Arabia Refining Ltd., last shut a hydrocracker unit at the Jubail refinery in late June-early July for a couple of days, traders have said earlier.